7 February 2015

phased array antenna

In antenna theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. Usually, the spatial relationship of the individual antennas also contributes to the directivity of the antenna array.  One common application of this is with a standard multiband television antenna, which has multiple elements coupled together and in radar applications.
Phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation. Simultaneous electrical scanning in both azimuth and elevation is also possible.
Publisher: Christian Wolff
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A phased array antenna is composed of lots of radiating elements each with a phase shifter. Beams are formed by shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction. 

The main beam always points in the direction of the increasing phase shift. Well, if the signal to be radiated is delivered through an electronic phase shifter giving a continuous phase shift then the beam direction will be electronically adjustable. However, this cannot be extended unlimitedly. The highest value, which can be achieved for the Field of View (FOV) of a planar phased array antenna is 120° (60° left and 60° right). With the sine theorem the necessary phase moving can be calculated.
One of the fundamental difficulties in designing a phased array is that significant portions of the wave power transmitted by one element of the array can be received by the surrounding array antenna elements. This effect, which is known as array mutual coupling, can result in a substantial or total loss of transmitted or received radar signal, depending on the coherent combination of all of the mutual-coupling signals in the array.
The amplitudes and phases of the array mutual-coupling signals depend primarily on the shape of the radiating antenna elements, the spacing between the array elements, and the number of radiating elements. There are as many different design possibilities for phased arrays as there are dozens of different radiating array elements to choose from, and the spacing and number of radiating elements can vary widely, depending on the scanning requirements.

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